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Procedure for REINFORCED MASS CONCRETE
1) The purpose of this Method of Statement is to defined and established methods on Mixing, Transporting, Casting, Consolidating and Curing of Mass Concrete in hot weather as per the approved IFC drawings.
1) Mass concrete: Any volume of concrete with dimensions large enough to require that measure be taken to cope with generation of heat hydration of the cement and attendant volume change, to minimize cracking (by ACI 116R).
2) Consolidation: The process of inducing a closer arraignment of the solid particles in freshly mixed concrete or mortar during placement by the reduction of voids, usually by vibration, centrifugation, rodding, tamping, or some combination of these actions; also applicable to similar manipulation of other cementitious mixture, soils, aggregates, or the like.
3) Cover: In reinforced concrete, the least distance between the surface of embedded reinforcement and the outer surface of the concrete.
4) Curing: Action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.
Please read also: PROCEDURE FOR WALL PRECAST CONCRETE OF MANHOLE -CIVIL
5) Heat of hydration: Evolved by chemical reactions with water, such as that evolved during
the setting and hardening of Portland cement, or the difference between the heat of solution of dry cement and that of partially hydrated cement.
6) Hydration: Formation of a compound by the combining of water with some other substance; in concrete, the chemical reaction between hydraulic cement and water.
7) Grade of concrete value of 28 days characteristic compressive strength in Mpa (N/mm2).
6. Matters Caused from Reinforced Mass Concrete
1) Cracking will be occurred due to thermal stress caused from temperature difference between external and internal due to hydration heat of concrete.
2). Sedimentation cracking will be occurred due to sinking quantity of concrete caused from placing heavy thickness of concrete structure.
3) Cold Joint will be occurred due to long duration of placing concrete caused from large quantity in characteristic of mass concrete.
4) Cracking of plastic shrinkage will be occurred due to large amount of shrinkage caused from large volume of concrete structure.
8. Methodology for Casting of Mass Concrete
1) Mixing design of Concrete
a) Pre-cooling should be carried out to lower temperature of concrete as per ACl-305R.
b) Aggregate should be managed with continuous sprinkling water because concrete temperature is changed ±1 deg c when aggregate temperature is to be ±2 degc.
c) Chilled water and ice should be used because concrete temperature is changed ±1 deg c when water temperature is to be ±4.
d) Low heat cement should be used because concrete temperature is changed ±1 deg c when water temperature is coming to ±s deg c.
e) Admixture such a Micro Silica and so on should be used to preserve proper temperature of concrete during placing concrete.
a) Fresh concrete temperature should be maintenance below 35°C at the time of placing according to ACl-305.1.
b) Site supervisor should consider that Concrete temperature is rising 2-4°C during transportation.
c) Considering awaiting time, placing time, interval of mixer truck, arrival time and so on, from concrete mixing at Batch Plant to placing finish at site should be completed within 90 min according to ACl-305
3) Casting for Mass Concrete
a) Considering placing height and work circumstance, generally placing height of 1 layer is proper with below 30 cm to prevent settlement and increase durability.
b) Concrete placing should be carried out up to 10~15 cm below top steel bar to prevent settlement crack and then continue to do place concrete in final layer.
c) Concrete placing regularly should be carried out considering placing sequence such as placing height and placing direction as per interval of concrete supplying plan to prevent cold joint.
d) Placing thickness of concrete should be done below 30 cm and concrete placing should be continuously carried out on previous layer within 90 min with plan of concrete placing.
a) Vibratory should oe selected as purpose to increase concrete density and concrete strength, remove unnecessary mix air and spread equally aggregate in accordance with structure characteristics considering impact of vibration.
b) Vibratory will select in accordance with structure characteristics considering impact of vibration
9. Curing Methodology
1) Curing Methodology
a) Curing should be kept for after placing concrete in case of Mass Concrete and site Supervisor should be be aware cracking caused from thermal losses in the early 24hours.
b) Atmosphere temperature, relative humidity, concrete temperature, windy velocity causing Crack of concrete surface should be considered in curing time.
2) Water Curing
a) Water curing is development of concrete strength as well as minimizing crack among Curing Methodology as per ACl-308.1.
b) Early curing: Keep moisture condition without rapidly thermal change for 24 hours after placing concrete to prevent cracking from temperature change.
c) Middle curing : keep curing for 7 days since placing concrete and/or up above 70% of the required strength and keep temperature of curing water below 10 0c.
d) Temperature of curing water keeps below 10 °c compared to surface temperature to prevent thermal crack from thermal change in hardening and placed concrete
e) Last curing: Need to keep management to prevent effect from dry shrinkage 7 days after placing concrete.
3) Curing Compound
a) Curing Compound will be applied to concrete structure that is difficult with carrying out water curing.
10. Safety Control
During work you have to follow the all safety procedure to avoid unexpected accidents related to mass or manpower as professional.
1) Administrative control shall be applied for use to reduce heat stress e.g. provision of recovery 2) All work forces shall wear personnel protective equipment (safety helmet. goggles, safety shoes, dust mask, etc.).
3) Safety patrol and all other Project vehicle available shall be utilized during working time in case of emergency or accident that may occur to inform the nearest first aid team for assistance and contact to Safety Department.
4) Equipment and Hand tools shall be in good working condition and checked before using. Electric powered equipment shall have all connections and cords properly insulated and are suitably earthed to avoid electrocution hazards.
5) Material hauling, handling, to and from heights shall be kept at a safe distance from the load, and the load lowered slowly.
6) All equipment and materials shall be located proper away from excavation area as per SA Construction Safety Manual 2011.
7) 15 minutes prior to knock off time, all working areas shall be clean of all debris and put in designated garbage areas provided for by the Safety Department.
8) Tool box meeting shall be performed daily before starting to work.
9) Backing siren is installed on all equipment to serve as warning signal prior to backing out the equipment.
10) All fireproofed areas shall be barricaded with warning tapes.
a) All personnel will be issued with the following personal Protective equipment and must be worn at all times.
• Safety helmets
• Safety boots
• Safety Spectacles
• Hand Glove
• Dust Masks
• Ear Defenders
• Respirators as Required
• Wellington Boots
• Cotton coveralls with full sleeves shall be worn on construction site
• Disposable coveralls